Carburization is the conventional process for case hardening of ferrous metals, e.g. steel. In gas carburizing, the steel is exposed to an atmosphere which contains components capable of transferring carbon to the surface of the metal from which it diffuses into the body of the part. After an appropriate amount of carbon has been transferred, the steel is removed from the furnace and rapidly quenched, whereupon those regions in which the carbon level has been raised become hard and wear-resistant.
A variety of atmospheres have been employed, but they share a number of features in common. They must not react with the steel to form oxides or other undesirable compounds. This requirement precludes the presence of oxygen, and more than small amounts of water or carbon dioxide. Second, they must contain a substance which can serve as a carbon donor to the surface of the steel. Most commonly this is carbon monoxide, but occasionally hydrocarbons or oxygenated organic materials are employed. Third, the atmosphere must activate the surface of the steel so that reaction with the carbon donor proceeds at an acceptable rate. Hydrogen is highly effective as an activator, and is invariably present in practical carburizing atmospheres. Atmospheres derived from a variety of sources have been employed, but the most commonly used one is the so-called endothermic gas produced by partial combustion of natural gas in air. It consists essentially of 40% nitrogen, 40% hydrogen and 20% carbon monoxide. It is usually necessary to add a small amount of another constituent, commonly natural gas, to raise its carbon potential.
The use of endothermic atmospheres has a number of disadvantages. An expensive and elaborate endothermic gas (endogas) generator which requires continuing maintenance and attention of an operator is needed. Furthermore, the gas generator cannot be turned on and off at will; once it is running it is necessary to keep it in operation even though the demand for the atmosphere may vary from a maximum load to zero. The endogas, and the natural gas required to produce it, are wasted during periods of low demand. Further, natural gas is not constant in composition, containing varying amounts of ethane, propane and higher hydrocarbons in addition to the main constituent, methane. Variability in natural gas composition causes substantial changes in the endogas produced, and gives rise to problems of control. Finally, burning increasingly scarce and expensive natural gas to produce an atmosphere is inherently wasteful of energy.
A more recent development in the production of carburizing atmospheres involves the use of inexpensive by-product nitrogen which is introduced into the carburizing furnace along with methanol. The latter is thermally decomposed to produce a mixture of hydrogen and carbon monoxide, which serves as a carburizing atmosphere. It is usually necessary to add another constituent, frequently natural gas, to raise the carbon potential of such atmospheres. However methanol is commonly produced from natural gas or petroleum, and as fossil fuel becomes scarcer and more expensive, this approach again represents a waste of valuable energy.
There have been attempts to use ethanol, which may be produced from renewable agricultural products, for the generation of atmospheres for carburization. For example, U.S. Pat. No. 2,673,821 describes the generation of a furnace atmosphere from a mixture of ethanol and water. Control of carbon potential and prevention of massive carbon deposition (sooting) is achieved by addition of water. However, blue-black surfaces are reported in the literature for the hardened pieces indicating that incipient sooting took place. Further, since no constituent other than ethanol containing a relatively small amount of water is employed for generation of the atmosphere, excessive cost and wastage are experienced.
British Pat. No. 816,051 describes in general terms a process whereby nitrogen is saturated with a volatile organic substance and passed into a heat-treating furnace to generate an atmosphere suitable for carburization. Although no details are given, it is stated that ethanol may be used in this process. However, in Traitement Thermique, 62 (1971) 35-45 published by Traitement Thermique, 254 Rue de Vaugirad 75740 Paris, France, the authors state that only methanol and acetone are suitable in this process. Ethanol is reported to produce gum in the exit port of the furnace and to cause only weak and irregular carburization.